In recent years, there has been a remarkable spread of mobile phones and PDAs (Personal Digital Assistant). It has become a common practice to incorporate a compact digital still camera unit and digital video unit in such devices. These devices are characterized by severe restrictions on the size and cost. Therefore, there has been generally used a small-sized image pickup device equipped with a smaller number of pixels than that in a normal digital still camera, and a single focus optical system including about one to four plastic lenses. However, the magnification of a single focus optical system is on the same level as that in visual observation, and therefore, only the object located close to the operator can be captured. In the image pickup device mounted on a mobile information terminal, technology is advancing rapidly toward a further increase in pixel level and functional level. In this context, there has been an intense demand for a compact variable-power optical system compatible with an image pickup element of high pixel level and capable of capturing the image of a subject located far away from a user, wherein this variable-power optical system can be mounted on a mobile phone and others.
An example of a variable-power optical system of compact configuration is found, for example, in Patent Literatures 1 through 3 which disclose what is called a negative-positive two-component variable-power optical system composed of a first lens group having a negative power and a second lens group having a positive power arranged in that order as viewed from the object side. Further, for the purpose of implementing a less costly optical system of compact configuration, the Patent Literature 4 discloses a negative-positive-negative three-component variable-power optical system wherein all lenses are made of plastics. One of the systems having been recently proposed is a negative-positive-positive three-component variable-power optical system which is considered as the most advantageous for implementing the compact configuration of an optical system. Especially the Patent Literature 5 discloses a variable-power optical system of compact configuration using only two through four lenses. The Patent Literature 6 discloses the same negative-positive-positive three-component variable-power optical system, wherein more than 50 percent of all the lenses are made of plastics to ensure enhanced productivity.
Patent Literature 1: Japanese Patent No. 3333473
Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2002-82284
Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2005-208566
Patent Literature 4: Japanese Unexamined Patent Application Publication No. Hei 05-323190
Patent Literature 5: Japanese Unexamined Patent Application Publication No. 2003-177314
Patent Literature 6: Japanese Unexamined Patent Application Publication No. 2003-50352